The knowledge of accurate pore pressure, fracture gradient and formation strength is crucial while drilling a well for the success of the drilling operation. Pore pressure and fracture gradient are also controlling input parameters in borehole stability modeling, well planning, design, and wellpath optimization. While there are no commercially available tools for measurement of pore pressure ahead of the drilling bit, methodologies have been developed to calculate pore pressure in logged intervals using resistivity and/or sonic logs.
A few methods have been disclosed in the art for obtaining information regarding other properties while drilling. For example, a method has been disclosed to determine the porosity of a formation from drilling response. U.S. Pat. No. 4,064,749 discloses a method for determining porosity of a formation from drilling response, wherein a bit is attached to the lower end of a drill string that is rotated while the downward force on said bit is controlled. The method comprises the steps of measuring the revolutions of the bit, measuring the depth of the bit in the borehole, measuring the weight on said bit, determining the tooth dullness of said bit, measuring the torque applied to said drill string, determining a reference torque empirically, and determining said porosity by combining said measurements and determinations using an equation.
U.S. Pat. No. 4,949,575 discloses a technique for performing a formation analysis that utilizes drilling mechanics measurements as the porosity sensitive input. It comprises deriving a drilling signal indicative of the resistance of the formation to being drilled by a drill bit; deriving a plurality of additional signals indicative of formation properties; and, in response to said drilling signal and to said additional signals, deriving volumetric analysis of the subsurface formation.
U.S. Pat. No. 4,876,512 discloses a method for determining at well sites swelling-clay content of shales and shaly sandstones by conducting surface area measurements. The samples are washed with a fluid having a water activity substantially less than that of water that may contain a soluble cation, and measurements of the sample's dielectric constant are made at a pre-selected frequency (1 MHz) for subsequent comparison to calibration curves, thereby obtaining a measurement of the swelling clay content of the formation.
In U.S. Pat. No. 5,282,384, Holbrook discloses an improved technique based on sound mechanical theories from well logs for calculating the pressure of fluid contained in a sedimentary rock which has been naturally compacted under the influence of gravity. The effective stress portion of the method encompasses both internal and external measures of rock grain matrix strain. Thus the same effective stress calibration can be applied equally well to externally measured rock thickness data and petrophysically measured rock porosity data. The power law effective stress-strain relationship for any sedimentary rock can be determined from the weighted average of the power law functions of the minerals, which compose that sedimentary rock. In the present invention, Holbrook method has been modified both in methodology and in data type to predict pore pressure and fracture gradient.
A problem often encountered when drilling wells in many parts of the world is narrow drilling margins which require great precision in both pore pressure and fracture gradient prediction in order to prevent any shale instability problem resulting in risk of lost circulation and/or gas kicks/blowouts. For example, in the Gulf of Mexico deepwater environment the drilling margin may be less than ±0.5 ppg in both pore pressure and fracture gradient prediction. Therefore, the accuracy needed in wave velocities acquired from seismic, LWD and/or wireline logs is very important from the drilling aspect in addition to other known petrophysical and reservoir engineering applications of velocity.
Cuttings produced during drilling represent a potential quasi-real time source of information that can be procured at the rig site. The use of cuttings has been limited in the past partly due to the difficulties in performing measurements on very small samples.
In “Rig-site and Laboratory use of CWT Acoustic Velocity Measurements on Cuttings”, by Nes, et al, Society of Petroleum Engineers Paper No. 36854, 1996, incorporated by reference herein in the entirety, there is presented the use of continuous wave technology (CWT) for measurement of acoustic phase velocities on cuttings using potentially portable equipment that is suitable for testing of small samples of cuttings, thus offering a new source of data that can be attained in quasi real-time at the rig site.
In order to calibrate seismic velocities, logging while drilling (LWD) and/or wireline sonic measurements, and, even more importantly, real time accurate formation wave velocity measurements, drill cuttings provide a potentially invaluable source of information.
There is a great need in the art for a method that makes it possible to accurately predict pore pressure and fracture gradient in real time measurements at the rig site. If such data were available it would also be useful for identifying high risk shallow water zones, optimizing mud weight, detecting shallow hazard zones, detecting abnormal pressure zones, determining formation strength for wellpath optimization and, in general, for obtaining the most trouble-free, cost effective drilling.